Slow Spin Relaxation in Dioxocobaltate(II) Anions Embedded in the Lattice of Calcium Hydroxyapatite

Dioxocobaltate(II)-anion in calcium vanadate apatite: partial ordering, magnetic anisotropy, and slow relaxation of magnetization

Polycrystalline ceramic and small single-crystal samples of Co-containing calcium vanadate(V) with apatite structure were prepared for the first time. The Co2+ ions enter the apatite trigonal channels formally substituting protons of the OH- groups and form separate O-Co-O atomic groups elongated in the c direction, Co being additionally weakly coordinated to an oxygen atom of a VO4 group. At a high Co content, the hexagonal apatite structure undergoes a triclinic distortion followed by partial ordering of the Co2+ ions. The dc magnetic data fit well to a model of a zero-field split S = 3/2 state with a large negative D of -22 to -25 cm-1, suggesting a strong easy-axis magnetic anisotropy. The ac susceptibility measurements below T = 10 K reveal a multichannel slow relaxation of the magnetization in non-zero dc field. The temperature dependence of the relaxation time can be described by an Orbach process with the remagnetization energy barrier Ueff being equal to experimentally determined 2|D|. Modelling of the electronic structure shows that, with a small increase of the crystal field strength, the high-spin Co2+ ion changes its ground state from one with an unquenched orbital moment L = 3 to a fully orbitally quenched one. Both states are characterized by easy-axis magnetization vectors directed approximately perpendicular to each other with a smooth rotation of the vector at intermediate crystal fields. The model explains the weak magnetic anisotropy observed in the triclinic single crystal as well as the earlier reported ability of the dioxocobaltate(II) ion to behave like a single-ion magnet with either a high Ueff of hundreds of cm-1 or a moderate one of tens of cm-1. To the best of our knowledge that represents the first instance of the conversion of the d-element ground electronic state from orbitally degenerate to non-degenerate by a slight variation of the crystal field.

A Dy-based single ion magnet in a SrLaGaO4 matrix: enhanced parameters in an expanded crystal lattice

Dy3+ doped SrLaGaO4 exhibits unusually slow relaxation of magnetization determined by two widely separated excited Kramers doublets with a second remagnetization energy barrier of 223 cm-1. This value considerably exceeds that for analogous Ca(Y,Dy)AlO4 in spite of the apparently enlarged Dy3+ coordination sphere.

Dy3+ single ion magnet in the extended inorganic solid Ca(Y,Dy)AlO4

Confined in the extended solid with a widespread K2NiF4 structure type, cation Dy3+ exhibits magnetic bistability under a zero field.

Tb-based silicate apatites showing slow magnetization relaxation with identical parameters for the Tb3+ and Dy3+ counter ions

Tb-diluted and Tb-rich apatite-type silicates with compositions Y7.75Tb0.25Ca2(SiO4)6O2 and Tb8Ca2(SiO4)6O2, respectively, exhibit field induced multiple slow relaxation of magnetization. The former reveals two slow relaxation paths, the latter only one with a longer relaxation time of several seconds. The relaxation features of the Tb-diluted one are comparable with those of analogue compounds, where Tb is replaced by Dy, as well as with those of a Tb-doped calcium phosphate apatite. The relaxation parameters of the Tb-rich compound virtually match those of the Dy-based analogue Dy8Ca2(SiO4)6O2. The latter represents the first instance of independence of magnetization relaxation on the nature of a paramagnetic rare-earth metal ion in single ion magnet like materials.

All-Inorganic Single-Ion Magnets in Ceramic Matrices

All-inorganic single-ion magnets representing paramagnetic ions incorporated in a crystalline diamagnetic matrix are reviewed. Key results and advantages of this approach in comparison with the common strategy based on molecular metal-organic complexes are considered, and some unsolved problems and future perspectives are discussed.

Manifestation of strong magnetic and giant Raman anisotropy in single crystals of Cu for H substituted strontium hydroxyapatite

The linear [O–Cu–O]⁻ reveals a strong easy axis magnetic anisotropy with a high energy of 4 × 10² cm⁻¹ probed by magnetization measurements and polarized Raman spectroscopy of a single crystal.

Impact of fluoride for hydroxide substitution on the magnetic properties of a Co-based single-ion magnet imbedded in the barium apatite crystal lattice

The atomic group O–Co–O persists in the apatite channel and retains a high energy barrier for magnetization reversal.